1. Field of the Invention
The present invention relates to a standard sample used for instrumental measurement of ASTM color with a photoelectric colorimeter (color and color difference meter) which is a color testing method for petroleum products such as various lubricating oils, petrolatum and microcrystalline wax, and a method of instrumental measurement of ASTM color of a petroleum product using said sample.
2. Prior Art
There are two color testing methods for petroleum products, that is, the testing method for ASTM color and the testing method for Saybolt color which are prescribed in JIS K 2580.
Of these, the testing method for ASTM color is applied to various lubricating oils and petrolatum, etc., and specifies the color of a sample with a chromaticity value ranging from 0.5 (light) to 8.0 (dark) generally obtained by comparing the color of the sample under test with the color of a standard colored glass by the use of an ASTM chromometer. The standard colored glass used in the above method is prescribed in its dispersion by the luminous transmittance using the XYZ colorimetric system and the CIE standard illuminants C and the chromaticity coordinates based on the RGB color system in accordance with JIS Z 8722.
However, in cases where the ASTM color is measured by the use of the ASTM chromometer, since the ASTM color is visually judged by a measurer and the specification of the measurement results is not quantitative (when the sample color is between those of two standard glasses, a symbol L is attached to the value of the standard glass having a darker color, while when the sample color is darker than 8.0, a value of D 8.0 is assigned thereto), the current situation is that a personal difference is apt to arise, bothering the measure in the judgement. This tendency is particularly remarkable when the sample color is dense or dark.
Under such circumstances, automatization of the color testing method without resort to visual measurement is eagerly desired at the present time.
In view of the above, an attempt was made by the present inventors to investigate whether or not a commercially available photoelectric colorimeter (measuring instrument) can be used for measuring the ASTM color.
A photoelectric colorimeter used must be one in which a favorable correlation with the testing method as prescribed in JIS K 2580 can be obtained in the entire range of ASTM color ranging from 0.5 to 8.0. Furthermore, a universal method of testing the color of material, especially that of liquid is desired.
In the perception of color with human eyes under natural light, the quantity of stimulus due to three primary colors including red, green and blue is an important factor. In measuring a color with an optical instrument, natural light is used as the light source and the quantity of stimulus is tristimulus values.
Accordingly as a testing method for ASTM color, a universal method close to the perception of color with human eyes based on the tristimulus values by an optical instrument is desirable, and also as a method of specifying the results of measurement, a quantitative specification system without the use of L or D color specification is desired. Therefore, the testing method based on the measuring methods for color of reflecting or transmitting objects recommended by Commission Internationale de l'Eclairage (hereinafter abbreviated to "CIE") and prescribed in JIS Z 8722 and also the quantitative specification method for measured results based on the color-specification method based on the X YZ color system as prescribed in JIS Z 8701 are desired.
In measuring the chromaticity with a photoelectric colorimeter, it is necessary to express the measured value of a sample obtained with a commercially available photoelectric colorimeter in terms of the ASTM color.
Therefore, it is necessary to obtain the correlation between the above two factors.
The result of comparison between the sum of optical densities (.SIGMA.D) based on the XYZ color system of a standard glass and the ASTM color of the same led to the finding of a correlation as represented by the following relationship formula: EQU A=.alpha..SIGMA.D+.beta.
wherein A is ASTM color, .SIGMA.D is sum of optical densities (DX+DY+DZ), and .alpha. and .beta. are each a constant for calibration of instrumental error.
Therefore, the measured value of the ASTM color is obtained by processing the measured X, Y and Z values in the operation-display section of the measuring instrument on the basis of the relationship formula.
As the above .SIGMA.D value varies with an instrumental error depending on an instrument used, it cannot always be constant even for the same sample. It is necessary, therefore, to prepare a standard liquid sample for each ASTM color to calibrate each instrument (photoelectric colorimeter) therewith.
Such a standard sample is required to comprise a base material having a low volatility, resistance to oxidation, low deterioration, such as oxidation, with the elapse of time and good color stability.